There has been proposed a signal reproducing system for reproducing information signals such as video signals and audio signals from a disc-shaped recording medium having record tracks in which the information signals are recorded with an alignment of pits. In the signal reproducing system, the record tracks on the recording medium are scanned by a light beam and variations which the light beam is subjected to are detected to reproduce the recorded information signals. Such a recorded information signal reproducing system is known as a video disc system, a digital audio disc system or the like. A reproducing apparatus for such a system is required to have automatic control operations respectively for maintaining the light beam, which is directed to the recording medium to scan the record tracks thereon, in correct tracking relation to each record track and for focusing the light beam properly on the record tracks while scanning the record tracks with the light beam. These automatic control operations for causing the light beam to impinge correctly upon the record tracks and for focusing the light beam properly on the record tracks are called tracking control and focusing control, respectively, and are usually carried out by driving an optical device, such as a lens or a mirror, partially constituting an optical head for causing the light beam to be directed to the record medium to control the position thereof with control signals which are obtained respectively in response to the tracking condition and the focusing condition of the light beam on the record medium.
In order to drive the optical device for such tracking control or focusing control, the reproducing apparatus is provided with a driving circuit which is operative to supply a driving current or voltage to an actuating device provided for the optical device in accordance with the control signal. As for driving the optical device in this way, it has been proposed to employ a control circuit operating with a pulse-width modulated signal which is operative to supply the control signal formed into a pulse-width modulated signal to the driving circuit with the intention of improving efficiency in driving.
A proposed control circuit which operates with a pulse-width modulated signal and employed for such a use as mentioned above is shown in FIG. 1 of the accompanying drawings. In the circuit of FIG. 1, an analog control signal Se as shown in FIG. 2A obtained for the tracking control is supplied to a terminal 1, and a triangular waveform signal Ss having a constant amplitude and a constant cyclic period, as shown also in FIG. 2A, is supplied to a terminal 2. The analog control signal Se and triangular waveform signal Ss are compared in voltage level with each other at a level comparator 3, and a pulse-width modulated signal S.sub.1 which is given a high level h when the voltage level of the analog control signal Se is higher than that of the triangular waveform signal Ss and a low level l when the voltage level of the analog control signal Se is equal to or lower than that of the triangular waveform signal Ss so as to be formed into a series of pulses each having the width varying in response to variations in the voltage level of the analog control signal Se, as shown in FIG. 2B, is derived from the level comparator 3. The pulse-width modulated signal S.sub.1 is supplied directly to one of the input terminals of a driving circuit 5 and also to an inverter 4. The pulse-width modulated signal S.sub.1 supplied to the inverter 4 is subjected to level inversion thereat so as to be formed into an inverted pulse-width modulated signal S.sub.2 as shown in FIG. 2C and this inverted pulse-width modulated signal S.sub.2 is supplied to the other of the input terminals of the driving circuit 5.
The driving circuit 5 is operative to supply a driving current to an electromagnetic coil 6 constituting an actuating device for driving the optical device, for example, the lens or the mirror as aforementioned, and comprises a pair of complementary transistors 7 and 8 having respective bases connected in common to the input terminal to which the pulse-width modulated signal S.sub.1 is supplied and another pair of complementary transistors 9 and 10 having respective bases connected in common to the input terminal to which the inverted pulse-width modulated signal S.sub.2 is supplied. The transistors 7 and 8 are connected between a power source +B.sub.0 and the ground with respective emitters coupled in common to make a connection point P.sub.1 and the transistors 9 and 10 are also connected between the power source +B.sub.0 and the ground with respective emitters coupled in common to make a connection point P.sub.2. The electromagnetic coil 6 is connected between the connection points P.sub.1 and P.sub.2.
With the configuration described above, when the pulse-width modulated signal S.sub.1 has the high level h and the inverted pulse-width modulated signal S.sub.2 has the low level l, the transistors 7 and 10 are made conductive so that the electromagnetic coil 6 is supplied with a first driving current flowing therethrough from the connection point P.sub.1 to the connection point P.sub.2, and when the pulse-width modulated signal S.sub.1 has the low level and the inverted pulse-width modulated signal S.sub.2 has the high level h, the transistors 8 and 9 are made conductive so that the electromagnetic coil 6 is supplied with a second driving current flowing therethrough from the connection point P.sub.2 to the connection point P.sub.1. Accordingly, each duration in which the first driving current flows through the electromagnetic coil 6 and each duration in which the second driving current flows through the electromagnetic coil 6 are varied in response to the variations in the voltage level of the analog control signal Se.
However, in such a proposed control circuit, control gain G is determined to be in proportion to the ratio of a voltage E.sub.0 of the power source +B.sub.0 to the amplitude (peak to peak value) of the triangular waveform signal Ss, and therefore the control gain G is undesirably varied to exert a baneful influence upon the current supply to the electromagnetic coil 6 by the circuit if the voltage E.sub.0 of the power source +B.sub.0 varies. Accordingly, it is necessary to provide a stabilized power source supplying a constant voltage as the power source +B.sub.0. In such a case, since the power source +B.sub.0 constitutes a power supply for the driving circuit 5 which requires large power, the stabilized power source which is able to supply large power is required and this results is a problem that a burden on the circuit construction is increased. Further, in the proposed control circuit described above, the electromagnetic coil 6 is supplied always with the first or second driving current through the driving circuit 5 when the control circuit operates, and consequently the electromagnetic coil 6 causes considerably large power consumption